In the field of magnetic memories, so-called M-RAM (Magnetic Random Access Memory) consisting of a magnetic tunnel junction have attracted considerable interest ever since the development of such tunnel junctions which have high magnetoresistance at ambient temperature. These random-access magnetic memories have many significant advantages:                speed (write and read times of just a few nanoseconds);        non-volatile;        no read/write fatigue; and        unaffected by ionising radiation.        
This being so, they are increasingly replacing memory that uses more conventional technology based on the charge state of a capacitor (D-RAM, S-RAM, FLASH).
In these magnetic memories, coding of information (“0” or “1”) depends on the relative orientation (parallel or antiparallel) of the magnetization of two magnetic layers having different coercivities, one of the layers being “free”, i.e. having a magnetization direction that can be modified by applying an external low-intensity magnetic field and the other layer being referred to as “anchored”, i.e. having a magnetization direction that is unaffected by said external magnetic field.
This change in the relative orientation of magnetization directions modifies the electrical resistance of the stack of two layers thus formed and the magnetic state is read by measuring an electric voltage after injecting an electric current in a direction that is perpendicular to the plane of the layers.
Generally speaking, information is written by sending two electrical pulses via conductors that intersect at a right angle close to the point where the memory cell in question is located. Adding the two magnetic fields created by these electrical pulses at the level of the cell and the direction of the electric currents injected makes it possible to change the magnetization direction of the “free” layer, thus writing the information in question.
However, the existence of relatively widely distributed switching fields of all the memory cells due to the method of fabrication makes it necessary, in order to ensure a change in the orientation of magnetization, to use an external magnetic field that is higher than the highest switching field of said distribution. This being so, there is a risk of inadvertently reversing certain memory cells located on the corresponding row and/or column having a switching field, possibly located in the lower part of the distribution, that is weaker than the magnetic field generated by the row or column alone.
If one wants to make sure that no memory cell can be written by one row or column alone, the write current must be limited so that it never exceeds, for these memory cells, the magnetic field that corresponds to the lower part of the distribution with the risk of not writing the selected memory cell at the intersection of said rows and columns if the switching field for that memory location is in the upper part of the distribution. In other words, this architecture with selection by magnetic fields using rows and columns of conductors can easily result in write addressing errors.
In addition, it has generally been observed that the average value of the switching field increases as the size of the memory cells decreases. This being so, a stronger electric current is required in order to ensure actual switching of the magnetization of the memory cell and this consequently entails an increase in the required electric power.
Because of this, another write technique referred to as “magnetization switching by spin-polarized current” has been proposed. This technology involves writing memory cells by using a spin-polarized electric current rather than an external magnetic field. In fact, it has been demonstrated that a spin-polarized current is capable of causing precession or even reversal of magnetization by spin angular momentum transfer between polarized carriers and the magnetic moment of the layer in question (see U.S. Pat. No. 5,695,864, for example).
One of the advantages of this technique is the fact that the same current row is used for both reading and writing the magnetic information and this simplifies the architecture of the device considerably. Thus, as it passes through the various layers of the magnetic stack in question, the electric current is polarized and the electron spin tends to align itself with the local magnetization direction. If there is no depolarization in the layers through which the current passes, this polarization is maintained in the second magnetic layer and in turn induces precession of the magnetization of the so-called “free” magnetic layer around the polarization direction.
If the electric current density increases, the angle of the precession cone increases until it eventually exceeds 90° for a certain critical current, thus causing the magnetization of the “free” layer to flip to a direction that is opposite to its initial direction.
Nevertheless, this particular technology is hampered by one serious limitation. In fact, in this configuration, in order to achieve magnetization reversal, it is necessary to overcome the demagnetizing field of said “free” layer. For thin magnetic films, this demagnetizing field tends to hold magnetization in the plane of said layer. Because this demagnetizing field is proportional to the magnetization of the material, it is obvious that magnetization reversal makes it necessary to inject a high-intensity current which is capable of damaging the device, especially by causing electric breakdown of the insulating barrier that separates the two magnetic layers in the case of a magnetic tunnel junction.
Magnetic thin-film systems are also used in the field of radio-frequency oscillators. RF oscillators have undergone considerable development directly associated with corresponding development of mobile telephony. In fact, mobile telephony has brought about the use of oscillators having a very wide frequency band with especially good jitter performance and hence a high quality factor.
One technical solution to meet this demand is to use electron-spin based radio-frequency oscillators. Using such oscillators makes it possible to obtain a wide frequency band with a high quality factor Q and straightforward frequency tunability and, moreover, to use a relatively simple architecture.
Spin polarization of an electric current which causes magnetoresistive phenomena in magnetic multilayers such as giant magnetoresistance and tunnel magnetoresistance is known. In addition, when it passes through a magnetic thin layer, such a spin-polarized current can affect the magnetization of a magnetic nanostructure by inducing reversal of its magnetization in the absence of any external magnetic field or by generating sustained magnetic excitation also referred to as oscillations. The frequency of this excitation depends, in particular, on the density of the current that flows through the nanostructure.
Using the effect of generating sustained magnetic excitation in a magnetoresistive device makes it possible to convert this effect into electrical resistance modulation that can be directly used in electronic circuits with the consequent possibility of acting at the level of frequency.
However, one of the problems encountered with these radio-frequency oscillators is the density of the spin-polarized current that has to be injected into the magnetic system in question and which is capable of causing damage to the device due to breakdown or electromigration phenomena.
Regardless of the prospective application, in order to reduce the current densities required to write information, attempts are always made to obtain a thin-layer magnetic material whose magnetization is spontaneously parallel to the plane of that layer but can easily be oriented in a perpendicular direction by the effect of a low-amplitude magnetic field (or polarized current) or a thin-layer magnetic material whose magnetization is spontaneously (without any external magnetic field or polarized current) perpendicular to the plane of that layer.
For this purpose, the reader is reminded of the physical principles that underlie these phenomena. For a single magnetic layer, i.e. for example a thin layer of magnetic material deposited on a substrate where there is no particular interaction with said layer, the form effect (the fact that the lateral dimensions of this layer are much larger than its thickness) tends to keep its magnetization direction in plane (so-called “planar” magnetization).
If a magnetic field of increasing amplitude is applied in a direction perpendicular to the plane of this layer, the direction of its magnetization will gradually exit this plane and orient itself parallel to the applied field. It will therefore be perpendicular to the plane when the applied magnetic field reaches a perpendicular saturation field value Hsp equal to a so-called “demagnetizing field” Hdm that is proportional to the magnetization per unit of volume Ms of this magnetic material in accordance with the following equation:Hsp=Hdm=4πMs.
To give some idea of values, this field Hsp is of the order of 18 kilo-oersteds (kOe) for a material such as cobalt and 6 kOe for nickel. The first way of reducing this field Hsp is therefore to use a weakly magnetic material. However, this may be disadvantageous for some applications in which the wanted signal depends on this magnetization.
A second way of reducing Hsp is to introduce an additional term of opposite sign to Hdm. This so-called “perpendicular anisotropy” term Hap may, as indicated in the rest of this explanation of the prior art, be the result of volume anisotropy of magnetocrystalline origin or induced by elastic growth strains or it may be interface anisotropy due to interfacial electronic interactions. The influence of a layer of platinum in contact with a magnetic layer of cobalt, nickel or iron is a typical case, for example.
When this additional term is present, the perpendicular saturation field can be expressed as follows:Hsp=Hdm−Hap.
Qualitatively, the perpendicular saturation field Hsp will therefore reduce uniformly as Hap increases, the magnetization of the magnetic layer always being parallel to the plane until it approaches zero, the limit beyond which, when Hap exceeds Hdm, magnetization of the magnetic layer will spontaneously (i.e. without any applied magnetic field) be perpendicular to the plane of the layer.
It must also be noted that, in the case of perpendicular anisotropy of interfacial origin, Hap will, as an initial approximation, be inversely proportional to the thickness e of the magnetic layer in accordance with:Hap=C+Kap/ewhere C is a constant that depends on the volume properties of the magnetic layer and where Kap, the perpendicular anisotropy constant, depends on the intimate structure of the material in contact with the magnetic layer and the structural quality of the interface.
This dependence of the perpendicular anisotropy field on the thickness of the magnetic layer therefore indicates that it will only be possible to stabilise magnetization in a direction that is perpendicular to the plane for thin magnetic-layer thicknesses and, conversely, that the critical perpendicular/planar transition thickness increases with the amplitude of Kap.
The first object of the invention in relation to applications of the RF oscillator or MRAM memory type is to propose a means of producing a magnetic layer with magnetization perpendicular to the plane of that layer which can be integrated in spin valve or tunnel junction type structures having free and anchored layers with planar magnetization. This additional magnetic layer with perpendicular magnetization is intended to be used as a “polarizer” (see U.S. Pat. No. 6,532,164).
In such a “polarizer”, the spin of the current electrons injected into the magnetic system is coupled with magnetization in a direction perpendicular to the plane of the layers and the axis of the magnetization precession cone is therefore also perpendicular to this plane. For weak currents, magnetization of the “free” magnetic layer rotates in a plane that is practically identical to the plane of the layers.
The use of synchronised current pulses and the uniaxial planar magnetic anisotropy of the “free” layer make it possible to reverse the magnetization direction easily by causing it to perform a half precession cycle in the plane of the layer.
The use of such a polarizer in the production of radio-frequency oscillators is also especially sought-after. In such a configuration, the spin-polarized current is injected continuously through the stack rather than as pulses. This being so, the precession motion of the magnetization is sustained rather than resulting in half precession for a write operation in the case of a magnetic memory.
If the magnetization that precesses is that of the free (or soft) layer of a tunnel junction deposited on top of the polarising layer, this precession movement results in oscillating variation of the resistance of the stack due to the tunnel magnetoresistance effect of the junction. This results in the appearance of an oscillating voltage between the two opposite-facing surfaces of the stack, this voltage can be used to produce a tunable radio-frequency oscillator with the frequency being directly related to the intensity of the injected current.
It is important to note that, in order for them to operate, the magnetic layers with perpendicular magnetization must not contain any materials that have a strongly depolarizing effect on the electrons in the vicinity of the active area of the structure.
By way of example, in the case of the perpendicular polarizer mentioned above, inserting a thin layer of platinum between this polarizer and the two magnetic layers of the spin valve or magnetic tunnel junction type structure would completely destroy the polarization of electrons brought about by this polarizer. In the rest of this document, the term “effective” magnetic thickness will be used to denote the thickness of the magnetic layer with perpendicular magnetization, considered relative to the direction of travel of the electrons, beyond any final layer of strongly depolarizing material such as platinum, palladium or gold.
Another object of the invention in relation to MRAM type applications is to propose a means of producing thin magnetic layers that can be integrated in spin valve or magnetic tunnel junction type structures having perpendicular magnetization where the magnetizations of the two active magnetic layers (anchored layer and free layer) are perpendicular to that plane.
A third object of the invention is to propose a means of producing a magnetic layer with planar magnetization, i.e. magnetization located in the plane of the layers that constitute it, for which its demagnetizing field is partially compensated by a perpendicular anisotropy term, thus making it possible to reduce the density of the current required to switch the magnetization of that layer. This magnetic layer may, for instance, be used as a free layer in spin valve or tunnel junction type structures with planar magnetization.
Various methods have been proposed in order to produce thin magnetic layers with magnetization perpendicular to their plane and capable of being used in some of the types of applications mentioned above.
Producing cobalt/nickel multilayers by vapour deposition on a buffer layer of gold covering the substrate has been proposed (Daalderop, Kelly and den Broeder, Physical Review Letters 68, 682, 1992). The operating window is relatively narrow (for example, for a cobalt thickness of 0.4 nm, the nickel layers must be 0.6 to 0.8 nm thick). Not only that, according to the authors, the result obtained depends critically on preparation conditions.
Adopting a similar approach, Ravelosona et al (Physical Review Letters 95, 117203, 2005) have proposed a combination of (cobalt/platinum)/(cobalt/nickel) multilayers, also prepared by vapour deposition. In this case, the effective magnetic thickness (i.e. above the final layer of platinum) is extremely small and equivalent to approximately 1.0 nm of cobalt.
In both these cases it seems necessary to grow the magnetic layers by vapour deposition, a technique that is not very compatible with industrial fabrication. The reason for this is that this perpendicular magnetic anisotropy property is due to the effects of elastic strain between the layers of nickel and cobalt which have crystalline parameters that are slightly different. This explains both the need to use such a preparation technique as well as the difficulty in producing such structures. In any case, any possibility of production on an industrial scale using this technology can be ruled out, at least at acceptable cost. In addition, these elastic strain effects only occur for certain crystalline magnetic materials. There is therefore no possibility, for example, of using other magnetic materials or amorphous magnetic alloys.
U.S. Pat. No. 6,835,646, which deals with structures of the substrate/buffer layer/Ni/FeMn/Cu type, proposes a method whereby growth of the nickel must be epitaxial. This means that the layers that are successively deposited must adopt the symmetry and inter-atomic distance of the underlying layers. In addition, the buffer layer must be made either of monocrystalline copper with crystallographic orientation (002) or diamond with crystallographic orientation (001). This can only be obtained by growth on a monocrystalline silicon substrate with crystallographic orientation (001) and chemical cleaning, moreover, in order to obtain satisfactory orientation of the copper or diamond buffer layer.
This production method is especially onerous to use because of the epitaxial growth and the monocrystalline nature of the substrates. In addition, no magnetic material other than nickel would give the hoped-for result.
Nishimura et al (Journal of Applied Physics 91, 5246, 2002, U.S. Pat. No. 6,844,605) have proposed another production method using structures based on rare earth metals of the GdFeCo/CoFe/Al2O3/CoFe/TbFeCo type, with “effective” thicknesses of magnetic metal (cobalt-iron alloy) of the order of 1 nm.
This production method involves using alloys based on metals in the rare earth family (gadolinium, terbium) that are known to be highly polluting and are prohibited in the industry.
It is evident from the foregoing considerations that none of the proposed solutions can be used to produce, using conventional magnetic materials and a simple preparation method, thin layers with magnetization perpendicular to their plane and having a sufficient “effective” magnetic thickness for the applications in question.
In fact, the magnetic thicknesses that one manages to achieve are either too small to provide exploitable polarization of the electric current that flows at right angles to the plane of the layers or it is necessary to use a specific magnetic material deposited using a very special method in order to achieve larger magnetic thicknesses.